As dicussed at http://forums.parallella.org/viewtopic.php?f=13&t=1053&sid=2d28ee29b5dd3c591d947074f46ac752&p=6654#p6654, this code: int a; int c; void __attribute__((interrupt)) misc_handler (void) { a*= c; } Is compiled into code that uses an uninitialized register. As it turns out, the interrupt attribute is actually a red herring (as long as you use the default of (-mfp-mode=caller). The problem is that, after emitting the mask-loading instruction, mode switching emits the mode switch to the caller's mode which uses that mask *before* the load of the mask, thus using the register uninitialized. The mask loading instruction, thus rendered useless, is later deleted. The things with lcm is that we have an algorithm that can be a bit expensive, but we can process multiple entities at almost no extra cost. The epiphany needs to load constants to do its mode switching; these constants can be anticipated further up in the dominance graph. This can be modelled as having a different entity for each mask needed, the need for which is indicated at the same point as the mode switch itself. Because the mask load entities are not subject to transparency issues (except in the unfortunate case of abnormal edges), lcm can move the loads up in suitable dominator positions. The modes priorities on the epiphany are also such that the mask loads have a mode with the same or higher priority as the mask uses. Also, the mask loads have lowered numbered entities than the mask uses. As the lcm part of optimize_mode_switching inserts, for each priority, the mode setting in ascending order of entities, and insert_insn_on_edge appends to the currently registered sequence, this works find there. The segment-based code also preserves entity order when inserting before an instruction. However, when inserting after a basic block head, later inserted mode switch instructions end up prior to ones earlier inserted into the insn stream. To preserve the order, in the case of an initially empty basic block, what we have to do is append the new instructions at the end of the basic block.
Created attachment 32447 [details] patch The attached patch implements this aforementioned insertion at the end of an (initially) empty basic block. I'm currently bootstrapping/regtesting this on i686-pc-linux-gnu
Created attachment 32526 [details] preprocessed libjava file With the latest proposed patch, we get an assertion failure building libjava during the i686-pc-linux-gnu bootstrap; this is the command line: ./cc1plus -fpreprocessed interpret.ii -quiet -dumpbase interpret.cc -mtune=generic -march=pentiumpro -auxbase-strip .libs/interpret.o -g -O2 -Wswitch-enum -Wextra -Wall -version -fno-rtti -fnon-call-exceptions -fdollars-in-identifiers -ffloat-store -fomit-frame-pointer -fwrapv -fPIC -o interpret.s The block in question looks like this: (code_label/s 9087 9590 9090 17 990 "" [1 uses]) (note 9090 9087 9088 17 [bb 17] NOTE_INSN_BASIC_BLOCK) where the BB_HEAD is the CODE_LABEL, and the BB_END is the NOTE_INSN_BASIC_BLOCK. The caller of new_seginfo is the abnormal-edge code that I've patched to handle non-empty blocks differently; this block is mistaken for a non-empty block. Now, interestingly, the pre-existing code already handles this incorrectly, by inserting instructions between the CODE_LABEL an the NOTE_INSN_BASIC_BLOCK.
This patch: http://gcc.gnu.org/ml/gcc-patches/2014-04/msg00091.html has been approved for gcc4.10, modulo one spelling fix: http://gcc.gnu.org/ml/gcc-patches/2014-04/msg00263.html
Author: amylaar Date: Fri Apr 11 18:12:53 2014 New Revision: 209312 URL: http://gcc.gnu.org/viewcvs?rev=209312&root=gcc&view=rev Log: gcc: PR rtl-optimization/60651 * mode-switching.c (optimize_mode_switching): Make sure to emit sets of a lower numbered entity before sets of a higher numbered entity to a mode of the same or lower priority. When creating a seginfo for a basic block that starts with a code label, move the insertion point past the code label. (new_seginfo): Document and enforce requirement that NOTE_INSN_BASIC_BLOCK only appears for empty blocks. * doc/tm.texi.in: Document ordering constraint for emitted mode sets. * doc/tm.texi: Regenerate. gcc/testsuite: PR rtl-optimization/60651 * gcc.target/epiphany/mode-switch.c: New test. Modified: trunk/gcc/ChangeLog trunk/gcc/doc/tm.texi trunk/gcc/doc/tm.texi.in trunk/gcc/mode-switching.c trunk/gcc/testsuite/ChangeLog
Author: amylaar Date: Fri Apr 11 18:27:45 2014 New Revision: 209318 URL: http://gcc.gnu.org/viewcvs?rev=209318&root=gcc&view=rev Log: gcc/testsuite: PR rtl-optimization/60651 * gcc.target/epiphany/mode-switch.c: New test. Added: trunk/gcc/testsuite/gcc.target/epiphany/mode-switch.c
Fixed with commits of comment #4/#5.